The present invention relates to power supplies, an in particular, to an intelligent ballast for powering a lighting load, for example a gas discharge lamp such as a fluorescent lamp. The present invention relates to ballasts of the type disclosed in the Assignee's U.S. patent application Ser. No. 10/824,248 filed Apr. 14, 2004 and entitled Multiple-Input Electronic Ballast With Processor, the entire disclosure of which is incorporated herein by reference.
In the ballast disclosed in the above-identified pending patent application, the ballast includes an input or front end power circuit section that includes an RF filter and rectifier and a valley fill circuit including an energy storage capacitor, for providing a DC bus voltage. The DC bus voltage is provided to a back end or output stage including an inverter and an output filter. In the back end, an inverter is driven to provide a high frequency AC output voltage that is filtered by an output filter and provided as the voltage supply to the lighting load.
The ballast includes a processing section including a microprocessor which receives inputs, from both internal sources within the ballast itself and from external sources. For example, the internal sources of inputs may include an input voltage from the AC main supply, an input voltage from the DC bus concerning the DC bus voltage, an input concerning the output lamp current, and an input from the output voltage to the lamp. In addition, external sources of inputs to the ballast may include an external photosensor, an infrared receiver, a phase-control dimmer, and an analog voltage source. Furthermore, the processor has a communication port that receives information via the DALI or other communications protocol. DALI stands for Digital Addressable Lighting Interface and is described in an International Electrotechnical Commission document IEC 60929. The DALI communication port, microprocessor, and sensor input circuitry are powered by a power supply which receives rectified AC voltage from the output of the rectifying circuit.
In the above-described ballast, a fuse is placed to protect the ballast in the event of ballast failure, for example a power circuit short. However, if the ballast fails, and the fuse blows, the entire ballast fails including the processing section. This presents a problem because in the processing section handles incoming information from attached sensors and communicates this information to the communication link via the communication port for use by other system components. If the ballast fails and the fuse blows as a result of a fault in the power circuit section, it is undesirable to have the processing circuit portion also be without power. If the processing section is without power, then the information from any connected sensors is no longer available to the rest of the system. Thus, a single ballast failure in the power circuit portion can have far reaching consequences to the system if the ballast that fails is one that has a sensor connected to it.
It is therefore desirable to provide a ballast circuit such that, if a failure occurs in the portion of the ballast that supplies lamp power, only the power circuit section will be without power when circuit power is interrupted and the remaining processing portion that processes inputs from the sensors connected to the ballast, continues to operate.